The present invention relates to radio frequency (RF) communication systems, and more specifically, to apparatus and methods for rejecting receive band signals in RF mixed-signal amplifiers.
Wireless communication, such as cell phones for voice and data, has become extremely popular. Currently, several wireless schemes are in use, including GSM, TDMA, and CDMA. Of these, CDMA appears to be emerging as the standard in the U.S., European and Asian markets. CDMA often requires RF transmissions using both phase and amplitude modulation. The efficiency and power consumption of the power linear amplifiers used to generate an RF signal in either a CDMA cell phone or base station are therefore extremely important. Use of low efficiency linear amplifiers is detrimental for several reasons.
Such amplifiers tend to burn a significant amount of energy which is problematic, particularly in a battery operated cell phone. Power consumption is also problematic in base stations. The heat caused by many low efficiency amplifiers in a base station can cause components to fail, thus reducing reliability. The linearity of the power amplifier is also important. In a base stations where the transmission of multiple signals occurs simultaneously, amplifiers characterized by poor linearity may cause the inadvertent mixing of these signals.
A number of types of amplifier classes can be used in RF communication systems, including Class A, Class AB, Class C, Class E, Class F, and Class D (sometimes referred to as digital amplifiers). Each of these types of amplifiers, however, have significant problems when operating in the RF range. For example, Class A and Class AB amplifiers have very poor efficiency but reasonable linearity. Class C amplifiers are reasonably efficient but are only practical for phase modulation. Similarly Class E, F, and D amplifiers are typically only useful for phase modulation applications. Class E amplifiers have improved power efficiency when compared to C type amplifiers, but large voltage swings at their output limit their usefulness. Class F amplifiers exhibit relatively efficient switching characteristics with a repeating input signal. But with a non-repeating input signal, such as those normally encountered in a cellular phone or base station, the problems caused by harmonics become overwhelming.
Conventional class D amplifiers have linear operating characteristics and are generally highly efficient at lower frequencies but have heretofore been subject to several drawbacks at higher frequencies. Most notably, at higher frequencies such as RF they exhibit switching problems at their output transistors. As these transistors switch on and off rapidly, switching transients including high levels of current and voltage are developed at the output, causing overshoot and undershoot.
Another problem with conventional class D amplifiers when used in communication systems where RF signals are both transmitted and received is the xe2x80x9cleakagexe2x80x9d of energy from the transmit band into the receive band. This may occur if the duplexor or T/R switch at the antenna does not completely isolate the signals received at the communication device from the transmit circuitry within the device.
Most cellular systems today use frequency division duplexing (FDD) to achieve simultaneous transmit and receive capability. This is accomplished by using separate frequency bands for transmitting and receiving. For example, IS-95 CDMA systems in the United States uses 824-849 MHz for transmitting from a mobile station (i.e., upstream transmission) and 869-894 MHz for receiving at the mobile station (i.e., downstream transmission). FDD systems require limits on transmit emissions in the receive band to avoid corresponding degradation of the sensitivity of their own and neighboring mobile receivers. Systems which employ time division duplexing (TDD) also require limits on transmit emissions in the receive band, but typically to a lesser extent.
In view of the foregoing, an efficient amplifier capable of reducing transmit emissions in the receive band is needed. An amplifier capable of maximizing dynamic range in the transmit band to achieve maximum efficiency is also needed. A method for calibrating the transmit/receive band resonators so that the noise shaping capability is aligned to the transmit/receive bands is needed.
According to various embodiments of the present invention, a bandpass amplifier for use in a communication system is configured to reduce transmit emissions in the receive band and/or maximize dynamic range. The communication system has a transmit band and a receive band associated therewith. The bandpass amplifier includes a frequency selective network for noise shaping an input signal; an analog-to-digital converter coupled to the frequency selective network; a switching device coupled to the analog-to-digital converter for producing an output signal; and a feedback path for feeding back the output signal to the frequency selective network to facilitate the noise shaping. The frequency selective network includes first filtering circuitry for selectively passing the transmit band, and second filtering circuitry for selectively passing the receive band. The first and second filtering circuitry being configured to effect suppression of energy associated with the transmit band in the receive band.
One aspect of the present invention provides a bandpass amplifier having first and second signal paths. The first signal path includes a first number of transmit band resonators operable to resonate at the transmit band, and a second number of receive band resonators operable to resonate at the receive band. The second signal path including a third number of transmit band resonators operable to resonate at the transmit band, and a fourth number of receive band resonators operable to resonate at the receive band. The difference between the first and third numbers is equal to or less than two; and the difference between the second and fourth numbers is equal to or less than two.
According to a specific embodiment of the present invention, the bandpass amplifier further includes a signal generator for applying a test signal to a selected one of the resonators; a peak detector for detecting a signal strength of the signal passed through the selected one; and control circuitry for adjusting the selected resonator to maximize signal pass rate of the selected resonator at the corresponding one of the transmit and receive bands.
Another aspect of the present invention provides a method of calibrating a bandpass amplifier having a transmit band and a receive band associated therewith. The method includes receiving information representing the transmit band; calculating the receive band based on the information and a frequency offset between the transmit band and the receive band; and adjusting the first filtering circuitry and the second filtering circuitry to maximize signal pass rate at the transmit band and the receive band, respectively.
According to a specific embodiment of the present invention, the adjusting process includes selecting a filter which is to be calibrated among the first and second filtering circuitry; applying a signal to the selected filter; detecting signal strength of the signal which passes through the selected filter; and tuning the selected filter in response to the signal strength.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.